This invention relates to fluid flow measurement and, more particularly, to a method and system for measuring fluid flow in cylindrical pipes.
Over the years, many methods and apparatus have been devised for the measurement of volumetric fluid flow rate in closed conduits, particularly large diameter conduits. These known methods have included the use of venturi meters, tracers, and other techniques each of which has certain known disadvantages involving errors, complexity, and the like. One of the big sources of error lies in the fact that fluid flow in large diameter pipes (typically above 10 inches) almost invariably is turbulent. It is therefore difficult to measure accurately flow rate or flow velocity. Several of these methods are described in U.S. Pat. No. 3,564,912 issued Feb. 23, 1971 to Malone et al. and U.S. Pat. No. 3,918,304 issued Nov. 11, 1975 to Abruzzo et al.
The Malone et al. patent describes an apparatus utilizing numerical analysis techniques for determining volumetric flow rate that is particularly suitable for measurements in large diameter pipes. As described by Malone et al., four velocity measurements are made on the fluid flowing in a pipe. The velocity measurements are made using upstream and downstream transducer pairs which project acoustic energy towards one another along selected, chordally located acoustic paths. The acoustic paths lie parallel to one another in separate flow sections or planes and at specified distances from the edge or from the center (the flow axis) of the pipe. The several fluid velocity measurements thus obtained are processed in a digital processor and each are multiplied by a predetermined weighting factor. The specified locations of the measuring planes, and the predetermined weighting factors are in accordance with a well known mathematical relationship utilized in the numerical solution of integrals known as the Gaussian Quadrature formula or technique. More than four paths may be utilized and, as a variation, the weighting factors and positioning designations may be selected in accordance with two other well known mathematical techniques known as the Chebycheff or Lobatto technique, respectively.
While the Gaussian and other forms of integration described by Malone et al. are excellent for almost any pipe shape or configuration, they have a number of severe disadvantages. Among these are that each path length measurement requires a different weighting factor which must be separately computed. This means that the processing equipment for determining flow rate must be unnecessarily complex in order to perform the several mathematical operations involved. Another disadvantage of the Malone et al. technique is that several different length measuring paths are required. This again complicates the required processing equipment. A further disadvantage of the Malone et al. patent is that the measuring paths near the pipe wall have disproportionate weighting factors and are subjected to increased scatter.
It is also known from an article entitled "Optimization of the Test Section Length in Integrating Flow Measurement Methods" by B. Pfau, Archive for Technical Measurements, Sheet V1246-1 (February 1973) that by locating the measuring path in an eccentric sectional plane certain advantages are obtained. Among these advantages are that if the measuring plane is located at 0.493R, where R is the the radius of the conduit, the resulting measurement is independent of Reynolds number for both laminer and turbulent flow profiles. Thus turbulence is essentially removed as a source of error according to Pfau. But even with this improved accuracy afforded by Phau, many errors are still present particularly in measuring turbulent flow which almost always occurs in large diameter pipes.
Accordingly it is an object of this invention to obviate many of the disadvantages of the prior art flow rate measuring techniques.
Another object of this invention is to provide an improved method for determining volumetric flow rate in cylindrical conduits.
An additional object of this invention is to provide an improved system for determining volumetric flow rate in cylindrical conduits which system is relatively simple and of low cost.